1. Field of the Invention
This invention relates to an image forming apparatus, such as an electrophotographic copier, a printer or the like.
2. Description of the Related Art
In a conventional image forming apparatus, a toner image is formed on an image bearing member (for example, a photosensitive member) using an electrophotographic method or the like. The toner image is then transferred to a recording material in order to form an unfixed toner image thereon and the recording material is passed through a fixing device, comprising a fixing roller, a pressing roller and the like, to fix the toner image on the recording material.
In order to prevent so-called offset wherein toner adheres to the fixing roller, some fixing devices are provided with a surface releasing layer comprising fluororesin (for example, PFA, PTFE) or the like, coating the fixing roller. By coating the surface releasing layer on the fixing roller, offset of toner to the fixing roller due to a pressing force can be prevented.
However, the surface releasing layer comprising fluororesin or the like may generate triboelectric charges in the recording material so as to result in electrostatically-produced offset. More specifically, when a recording material bearing toner is charged to a positive polarity and a surface releasing layer generates triboelectric charges, the surface releasing layer becomes charged to a negative polarity. As a result, the surface releasing layer and the toner achieve opposite polarities and attract each other so as to result in electrostatically-produced offset.
In order to overcome this problem, a conductive substance may be incorporated within the surface releasing layer in order to reduce the potential of the surface of the fixing roller. Although using greater amounts of conductive substances effectively reduces the potential of the fixing roller surface and reduces the effect of electrostatic offset, the large amounts of conductive substance which is required reduces the releasability of the fixing roller surface, thereby enhancing pressure-generated offset.
In copending U.S. patent application Ser. No. 618,399 (filed Nov. 27, 1990), the assignee of the present application has proposed a method of positively preventing toner offset by applying a bias voltage to a core bar of a fixing roller in order to form a repulsive electric field between the surface of the fixing roller and toner. In this method, however, when a recording material enters a pressure-contact region, formed by the fixing roller and a pressing roller, in a state wherein the fixing roller and the recording material are greatly separated, offset could not be effectively prevented.